1. Field of the Invention
The present invention relates to overvoltage protectors and, in particular, relates to a low voltage surge arrester utilizing a semiconductor core to provide a predetermined surface creepage path which may be utilized in combination with a gas-filled electron tube type arrester.
2. Description of the Relevant Art
The art abounds with devices specifically designed for the protection of electronic equipment, which may be subject to overvoltage caused by lightning, high voltage line surges, external high voltage line contact, and the like. These devices utilize either separately or in combination, e.g. air gap arresters, gas tubes, carbon piles, metal oxide varistors, semiconductor devices, etc. These devices are utilized either separately or in combination in an attempt to provide instantaneous protection of the electronic equipment. However, each device has an inherent time lag either before it is able to break down or effect the reduction of the excess voltage. In addition, each type device has its own characteristic with regard to the amount of current it can safely bypass without becoming destroyed. Consequently, when sustained high voltages and currents are present, supplementary devices are required, e.g. a fusible link may be utilized to provide a short circuit until the fuse is vaporized, a shorting bar may be utilized, and when activated is applied across the load to handle the excessive surge currents, etc. Generally, these supplementary devices are expendable and require replacement once utilized.
The gas tube arrester has proven rather satisfactory for most applications, however, since it electrically responds only to voltages generally greater than 70 volts and it has an inherent time lag before igniting, other devices have been utilized in combination therewith to absorb any surge voltage or current until the gas arrester can absorb the major portion of the voltage surge.
A typical electrical overvoltage surge arrester is disclosed in U.S. Pat. No. 4,100,588 issued to Kresge on July 11, 1978. The arrester includes an insulating housing with end terminals and a plurality of varistors disposed within the housing and electrically connected between the terminals. The varistors are provided with a heat transfer and sinking collar which is electrically insulating and thermally conducting. The collar preferably is in thermally conductive contact with the inside wall of the housing to improve heat transfer to the housing. The collar configuration is such that when it is installed in the housing a passageway is provided therethrough, so that a longitudinal air space exists for the accommodation of arcing and for the venting of gas in the event of an arrester failure.
German Offenlegungsschrift No. 23 55 426 issued to Peche on May 15, 1975, discloses an overvoltage suppressor which includes two cupped shaped electrodes hermetically sealed to the ends of a tubular housing. The housing is made from a semiconductor material, preferably an oxide of zinc, titanium, copper and/or iron, which is doped with an oxide of bismuth or cobalt. The housing itself is filled with a noble gas.
It is an object of the present invention to provide a low voltage surge arrester that is capable of responding to overvoltage surges faster than devices known heretofore.
It is a further object of the present invention to provide a low voltage surge arrester that may respond to overvoltage surges in less than 1 micro-second.
It is another object of the present invention to provide a low voltage surge arrester which is small in size, economical to manufacture, and has the capability of protective response at relatively low voltages.
It is another object of the present invention to provide a low voltage surge arrester which has a relatively low firing voltage that is repeatable within narrowly defined limits and is capable of handling high surge currents.
The foregoing and other objects and advantages will appear from the description to follow. In the description reference is made to the accompanying drawing which forms a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. This embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.